This invention relates to the field of electronic packaging. More specifically, the invention relates to efficient heat transfer systems, particularly useful for dissipating heat from a power source, such as an integrated circuit chip in an electronic structure, to a heat sink.
In semiconductor technology, as applied to integrated circuit chip devices, it is important to provide suitable means to dissipate the heat that is generated within the chip so as to minimize the possible adverse effects of overheating, such as dimensional variations, variable operating characteristics and differential thermal expansion. Ineffective cooling of such devices results in decreased permissible power density, circuit density and system speed.
Whenever it is desired to dissipate heat from a source to a sink by conduction, an important aspect of the transfer is the amount of actual contact between the source and the heat sink. Regardless of the smoothness of the abutting surfaces and the amount of pressure that is employed to urge the surfaces together, there inevitably are irregularities that create air gaps between the adjoining surfaces. The presence of these gaps in the interface tends to increase the resistance to the flow of heat from the source to the sink inasmuch as air is considered a poor thermal conductor. The thermal resistance of an interface between two surfaces can be reduced by providing an interface material which fills the air gaps and voids in the surfaces. Many different structures have been used to transfer heat between two surfaces. Examples used in electronic packaging include adhesives that attach O-rings or heat sinks to chips, greases disposed between the surfaces, solder directly attaching the surfaces, and deformable pads clamped between the surfaces. In every case, the desire is to minimize the interfacial thermal resistance and to maximize the thermal conductivity of the transferring medium. Typical values for the effective thermal conductivity in electronic packaging are 0.2 to 2 W/m-K (SI Units).
An ideal medium for transferring heat from one surface to another should have low interfacial thermal resistance and high conductivity. Liquids have low interfacial resistance because they wet a surface forming a continuous contact with a large area. Most liquids do not, however, have very high conductivity. Solids, and in particular metals, have very high conductivity but high interfacial resistance. Most common heat transfer materials combine highly conductive particles with a liquid or plastic in order to exploit both characteristics. Examples of the former are greases and gels while the latter include filled epoxies, silicones and acrylics.
Greases have been developed that have conductivities as high as 3.8 W/m-K which is significantly better than the corresponding conductivities of filled adhesives. Typical problems with greases include pumping and dry out, both of which can cause the conducting medium to lose contact with one or both of the heat transfer surfaces. Pumping occurs when the structure is deformed, due to differential thermal expansion or mechanical loads, and the grease is extruded. The oils contained in a grease can be depleted by evaporation or by separation and capillary flow.
Liquid metals, such as mercury and alloys of gallium, potentially offer both low interfacial resistance and high conductivity. Mercury is a well-known biological hazard, making it unsuitable for high-volume commercial use; however, this is not the case with gallium and its alloys. Several alloys of gallium with very low melting points have been identified as potential liquid metal interface materials. The most important properties of these alloys are tabulated below. Both solidus and liquidus temperatures are listed for non-eutectic alloys. The conductivity is typically about 20 times better than adhesives, such as GE 3281 silicone currently in use.
The safety data sheet for the Galinstan alloy and the toxicological certificate from Eberhard-Karls University, Tubingen, xe2x80x9cconfirm that there is no danger to man or the environment.xe2x80x9d It is commonly used as a mercury replacement in thermometers and tilt switches.
There are several U.S. patents that teach the use of a liquid metal to transfer heat from a chip to an O-ring or heat sink. While these teach the containment of the liquid metal by an adhesive, none of them teach the use of an expansion chamber or a seal. Some of these patents are listed below.
U.S. Pat. No. 4,915,167 xe2x80x9cThermal Coupling to Enhance Heat Transferxe2x80x9d, 1990.
U.S. Pat. No. 5,198,189 xe2x80x9cLiquid Metal Matrix Thermal Pastexe2x80x9d, 1993.
U.S. Pat. No. 6,019,509 xe2x80x9cLow Melting Galluim, Indium and Tin Eutectic Alloys and Thermometers Employing Samexe2x80x9d, 2000.
U.S. Pat. No. 6,343,647 xe2x80x9cThermal Joint and Method of Usexe2x80x9d, 2002.
The ability to contain an electrically conductive liquid within an electronic package presents significant challenges. The liquid must be reliably retained in its enclosure throughout the life of the package if shorting is to be avoided. In addition, air must be excluded from the space between the heat transfer surfaces if the effective resistance is to be minimized. This is difficult due to the volume expansion of the liquid and is exacerbated if the metal changes between the liquid and the solid state within the temperature range of the package.
The present invention describes how to accommodate the volume change of a heat transfer liquid and how to eliminate air from the cavity after it is filled with the liquid.
Disclosed herein is a structure that gives substantially lower effective thermal resistance between two surfaces. A liquid metal, such as gallium or an alloy of gallium and indium, is used. Such liquids have very low interfacial resistance and high conductivity. Furthermore, they are nontoxic and environmentally benign. The liquid metal fills a cavity that is defined by two substantially parallel surfaces and by a material, such as an epoxy or silicone, disposed between them. Changes in the liquid volume, and changes of state from liquid to solid, are accommodated by an elastomeric plug with a diaphragm that also allows filling of the cavity between the two surfaces. Vent holes may be added to aid in filling. Such a structure can have thermal conductivities as high as 30 W/m-K.
More specifically, the invention relates to a thermal coupling device for facilitating heat transfer between generally parallel surfaces having a temperature differential between the two surfaces. The device comprises a metal that is liquid over substantially the entire range of temperatures that form the temperature differential. It also includes a seal to contain the liquid metal between the two surfaces. The seal includes means for accommodating changes in the spacing between the two surfaces and volumetric changes of the metal. The seal typically comprises a flexible elastomeric or polymeric material. It can be in the shape of an O-ring seal that is retained in an annular opening of one of the surfaces. Alternatively, it can comprise a flexible membrane and a retaining ring to retain the membrane in an annular opening in one of the surfaces. In either event, the seal may contain or include one or more vent holes. The parallel surfaces can comprise a surface of a semiconductor chip and a surface of a heat sink or a heat spreader.
The invention also relates to an electronic structure that comprises an integrated circuit associated with a semiconductor chip wherein the chip generates thermal heat when in use. A heat absorber is used for removing the generated heat from an exposed surface of the chip. The heat absorber may be a heat sink or a heat spreader. The heat absorber is adjacent but spaced a small distance (typically about 0.9 mm) from the exposed surface of the chip. A metal that is liquid over substantially the entire range of temperatures that are encountered during normal use of the structure is utilized as the heat transfer medium. Examples of the metals are gallium and its alloys. A seal is used to contain the liquid metal between the two surfaces, said seal including means for accommodating changes in the spacing between the two surfaces and volumetric changes of the metal. This seal comprises a flexible elastomeric or polymeric material. The elastomeric or polymeric material may contain at least one vent hole. The seal may comprise an O-ring that is retained in an annular opening of one of the surfaces. Alternatively, it may comprise a flexible membrane, with a metal or hard polymeric retaining ring being used to hold the membrane in an annular opening of one of the surfaces.
The invention also relates to a method of transferring heat between two substrates, such as a semiconductor chip and a heat sink or heat spreader having a temperature differential between them, and having generally parallel, opposing, spaced apart surfaces. The method comprises the steps of
a) confining the space between the two surfaces;
b) introducing a liquid metal into the space between the surfaces;
c) displacing all of the air in the space; and
d) providing a flexible seal to contain the liquid metal and to permit expansion and contraction of the metal without affecting the spaced relationship of the two substrates. The seal comprises a flexible elastomeric or polymeric material, such as an O-ring seal that is retained in an annular opening of one of the surfaces. Alternatively, the seal may comprise a flexible membrane, and a retaining ring to hold the membrane in an annular opening of one of the surfaces. The seal typically contains at least one vent hole for displacement of the air. This vent hole is sealed after the air is displaced.